The focus of this resubmitted application is to determine the cellular site and mechanisms of reactivation of cytomegalovirus (CMV). Human CMV (HCMV) infections are life-long and the virus persists in a latent state in monocytes, although other cellular sites are also thought to exist. During blood transfusion and organ transplantation, HCMV reactivation can cause severe disease in immunocompromised individuals. Therefore, determination of the parameters (including types and numbers of cells, and immune responses) that permit HCMV transmission during blood transfusions (TT) and organ transplantation is clinically important. The transfer of latently infected monocytes is hypothesized to be the mechanism of HCMV transmission and reactivation during blood transfusions. However, the exact cell number or type of infected cells required for transmission during blood transfusions or organ transplantation is unclear. In addition, the processes involved in cellular activation that result in CMV reactivation from latency are also unknown. The applicants propose two specific aims to analyze these aspects of CMV transfusion/reactivation using a MCMV/mouse model. In the first specific aim, the investigator will identify the cellular sites of infection and latency in MCMV infected donor mice. For these studies, serial dilutions of purified leukocyte subsets will be made and the viral burden quantitated prior to transfusion into syngeneic and allogeneic recipients. Through the removal of specific leukocyte subpopulations prior to transfusion he will test the hypotheses that a) only latently infected monocytes are able to transmit MCMV, b) transmission of MCMV by transfusion is inefficient, and c) MCMV-infected blood depleted monocytes is CMV safe. The second specific aim has been refocused from the original proposal. In this part of the project, the investigator will utilize the optimized MCMV transfusion model described in the first specific aim to evaluate recipient factors that modulate the efficiency of MCMV reactivation. These factors include allogeneic leukocyte interactions mediated by CD4 and CD8 T-lymphocytes, NK cells, and the effects of g IFN. He predicts that allogeneic leukocyte interactions mediate reactivation of MCMV in the recipient and that g IFN inhibits this process.